The Structure of a Midlatitude Squall Line: A Case Study

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Abstract
On 22 May 1976 a well-organized squall line with a width of more than 100 km passed through the mesonetwork of the National Severe Storms Laboratory in central Oklahoma. This squall line was evolving from its mature stage to the decaying stage when it passed the network. A total of 81 rawinsonde observations were made at nine stations within the network. These, as well as other observations, were analyzed to depict the kinematic and thermodynamic structure of the squall line. Composite and objective analysis techniques were used to arrive at the time-averaged structure on a vertical plane transverse to the squall line. The resulting air flow relative to the traveling squall line is consistent with the distribution of the thermodynamic variables and radar reflectivity. A familiar upshear tilt of the updraft is clearly indicated; the upper-level divergence maximum is located ∼70 km to the rear of the low-level convergence maximum. An interesting feature revealed by the analysis is that the horizontal momentum is approximately conserved following air-parcel motions in the updraft before the air reaches the upper-level outflow layer. Consequently, the large momentum originating in the low-level inflow layer is carried upward and meets with the air entering from the rear at middle levels, thus producing a second local convergence maximum. The upper-level updraft associated with this convergence apparently sustains the broad structure of the squall line. Below this updraft, a strong downdraft exists which is fed by a potentially cool dry air that enters the system from the rear at middle levels and is cooled by evaporation. The results of the analysis are compared to the structure of the tropical squall line delineated by other authors and some similarities between them are pointed out, including an “onion” shape formed by the temperature and the dew point curves.

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